Heterodyne interferometers typically use two light beams with slightly different frequencies (e.g., frequencies that differ by a few MHz). The two light beams are normally directed down different paths, a reference path that has a fixed length and a measurement path that includes a reflection from an object being measured. After traversing their respective paths, the beams are combined, and frequency or phase information extracted from the combination allows precise measurement of the object""s movement.
One light source for a heterodyne interferometer is a Zeeman-split laser. A Zeeman-split laser can use a single laser cavity to produce a beam containing two frequency components with wavelengths a few MHz apart. The two components in the laser cavity have wavelengths that are highly stable, phase-locked, and circularly polarized with opposite phase delays making the polarizations of the two components orthogonal.
For visible light, a frequency difference of a few MHz such as in the beam from a Zeeman split laser is too small to permit beam dispersive elements such as prisms or diffraction gratings to separate the frequency components within a reasonable working distance (e.g., within 500 mm for beam diameters between 3 and 12 mm). Accordingly, a conventional separation technique converts the circular polarizations of the frequency components in the beam from a Zeeman-split laser into orthogonal linear polarizations, and a polarizing beamsplitter separates the frequency components according to their linear polarizations. Generally, the output optics of the laser includes a quarter-wave plate that converts the orthogonal circular polarizations into orthogonal linear polarizations, and a half-wave plate reorients the linear polarizations along xe2x80x9cSxe2x80x9d and xe2x80x9cPxe2x80x9d polarization directions for an interferometer""s beamsplitter that separates the components.
In practice, the polarizations of the frequency components inside a laser cavity depart slightly from being perfectly circular, and the polarizations of the frequency components after the wave plates may not be perfectly linear or perfectly orthogonal. In particular, the frequency component intended to have the xe2x80x9cPxe2x80x9d polarization will have a small xe2x80x9cSxe2x80x9d-polarized component, and the frequency component intended to have the xe2x80x9cSxe2x80x9d polarization will have a small xe2x80x9cPxe2x80x9d-polarized component.
The interferometer""s beamsplitter cannot distinguish two frequencies and instead (with ideal operation) separates components of the input beam according to polarization. As a result, the separated beams have a small frequency contamination respectively inherited from the xe2x80x9cSxe2x80x9d and xe2x80x9cPxe2x80x9d polarization components, which contain mixtures of frequencies before separation. Further, the interferometer""s beamsplitter in actual operation fails to perfectly separate the polarization components, leading to further leakage of light of the wrong frequency into each separated beam.
A heterodyne interferometer typically measures the velocity of an object from the Doppler shift of a measurement beam reflected from the object. When the measurement beam and a reference beam are ideal monochromatic beams of slightly different frequencies, a beat frequency that is equal to the difference between the frequency of the Doppler-shifted measurement beam and the frequency of the reference beam can be compared to the original frequency difference between the beams. The variation in the beat frequency indicates the velocity of the object. However, frequency leakage superimposes other oscillations on the beat frequency and thereby can introduce a cyclic error in measurements. Precise measurements thus require the polarizations of the frequency components to be very linear and very orthogonal, for the cleanest possible separation of monochromatic measurement and reference beams.
Another concern in heterodyne interferometers is removing heat sources such as the laser from the thermal environment of the separated beams. Generally, the interferometer optics must be thermally protected from the laser to reduce thermal disturbance of beam paths and measurements. The thermal protection usually means keeping the laser separated from the interferometer optics and generally requires a mechanism for remote delivery of the beam from the laser to the interferometer optics.
One technique for delivering the beam from the laser sends the beam through an optical window into a thermally protected zone containing the interferometer optics. Mirrors can guide the beam into the thermally protected zone. This technique has difficulties in that precise and stable control of the beam generally requires mounting the laser and the interferometer optics on the same very stable frame, while still keeping the interferometer optics environmentally isolated from the laser.
Another technique for delivering the beam is to send the beam including both linear polarization components into a single polarization maintaining (PM) optical fiber. However, currently available PM fibers only provide extinction ratios (i.e., ratios of the preserved polarization to the orthogonal polarization) of up to about 20 dB, whereas precision interferometers typically require extinction ratios greater than about 35 dB, often closer to 50 dB. Generally, current PM fibers have too much crosstalk between the linearly polarized components to provide the clean frequency separation that good measurements require.
Yet another delivery technique separates the linear polarization components and sends the separated linearly polarized beams down separate PM fibers for recombination inside the protected zone of the interferometer. The initially good polarizations of the frequency components out of the laser allows a clean frequency split for input to the two PM fibers, and the beam combiner or auxiliary polarizers can clean up the individual polarizations of the beams exiting the PM fibers to compensate for the lower than desired extinction ratios of current PM fibers. However, some high-precision interferometers require very precise parallelism (e.g., microradian precision) for the frequency component beams. Passive opto-mechanical mounts have difficulties in achieving and maintaining the required level of parallelism over variations in temperature and through the vibrations and shock typically encountered during shipping and integration of an interferometer.
In view of the above-described limitations of current systems, heterodyne interferometers need additional systems and methods for thermally separating a laser or other beam source from interferometer optics while still producing cleanly separated monochromatic beams that are highly parallel.
In accordance with an aspect of the invention, a beam containing left-handed and right-handed circularly polarized components of slightly different frequencies is sent from a laser or other beams source into an optical fiber. The optical fiber can be an isotropic optical fiber that is a single mode fiber for the wavelengths of the beam or single mode for a wavelength slightly longer than the beam""s wavelengths. A quarter-wave plate can change the left-handed and right-handed circularly polarized components of the collimated output beam from the fiber into linearly polarized components, and half-wave plate can be adjusted to rotate the linear polarizations to produce the desired xe2x80x9cSxe2x80x9d and xe2x80x9cPxe2x80x9d polarizations at the plane of the interferometer beamsplitter.
One specific embodiment is a system including a light source, an optical fiber, and a polarizations conversion system. The light source, which can include a Zeeman split laser, generates a light beam that contains a first component having a left-handed polarization and a first frequency and a second component having a right-handed polarization and a second frequency. The optical fiber receives and conducts the light beam including the left-handed and right-handed components to a desired location such as a thermally protected zone of a heterodyne interferometer. The polarization conversion system, which is at an output of the optical fiber, converts the left-handed polarization of the first component to a first linear polarization and converts the right-handed polarization of the second component to a second linear polarization that is orthogonal to the first linear polarization. The optical fiber can be an isotropic fiber that is a single-mode fiber or a few-mode fiber for light having the first frequency and light having the second frequency.
Polarizations in the beam exiting the optical fiber generally depend on the bends and the temperature of the optical fiber, and the stability of the polarizations depends on the fiber being in mechanical and thermal equilibrium. The optical fiber can include a coating that reduces time for the optical fiber to reach mechanical equilibrium and/or a rigid jacket that resists dynamic bending. In the polarization conversion system, a quarter-wave plate can be mounted with an adjustable tilt angle and an adjustable roll angle to permit orienting the quarter-wave plate according to the properties of the left-handed and right-handed polarizations exiting from the optical fiber.
Another embodiment of the invention is a method for providing a heterodyne beam. The method includes: generating a light beam that contains a first component having a left-handed polarization and a first frequency and a second component having a right-handed polarization and a second frequency; coupling the light beam into an optical fiber; and converting the left-handed polarization and the right-handed polarization in the light beam exiting the optical fiber into respective linear polarizations, which are typically orthogonal to each other. A quarter-wave plate can convert the polarizations. A tilt angle and a roll angle of the quarter-wave plate can be adjusted to compensate for the effects that bends in the optical fiber have on the properties of the left-handed and right-handed polarizations exiting from the optical fiber.